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Questions tagged [fft]

FFT is an acronym for Fast Fourier Transform. Use this tag for FFT-related questions. Try to also select a specific chip or software tag when possible.

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What should I make of this spectrum analyzer data?

Here I have two signals that I placed into a spectrum analyzer. Channel 1 (yellow) comes from an avalanche photodiode + TIA circuit that has a 50 Ω output resistor, which is directly connected via BNC ...
Collin Fan's user avatar
1 vote
2 answers
81 views

Understanding dBm value of FFT measurements

I am getting familiar with the FFT functionality of my oscilloscope. I am using a Siglent SDS2204X Plus that has been firmware upgraded to 350 MHz. Testing of the FFT function involves the use of a ...
mike's user avatar
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1 vote
2 answers
64 views

Beat frequency - Square signal

This question comes from this one, LTspice - Beat frequency: It appears that when I did a FFT of two sinusoidal signals, there was no what is called beat frequency. When I do the same exercice with ...
Jess's user avatar
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1 answer
93 views

Does the FFT in LTspice not work properly?

I wanted to do an FFT of a square signal in LTspice. Here is the simulation: Here is the result of the FFT through R4: There is no reason to have frequencies below 100 kHz according to theory. What ...
Jess's user avatar
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0 answers
41 views

How can I determine how much power is in a frequency range via FFT in LTspice?

When using LTspice, I can see the FFT of a voltage. How can I see how much power is in a particular frequency band, and what portion of the total power it is? The FFT in LTspice is in dB, but it's ...
SRobertJames's user avatar
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0 answers
54 views

Distinguishing light sources with STM32 ADC and fast fourier transform

I want to identify if there are other light sources present at the same time as a 5 kHz 50% duty cycle PWM LED is on (this is arbitrary and it can be modulated at a different frequency). I have an ...
donut's user avatar
  • 325
0 votes
1 answer
35 views

Analysis for the peaks in FFT

Suppose I have 2 cases: 1st MOSFET switches with rise times and fall times of around 10ns (Vds). 2nd MOSFET switches with rise times and fall times of around 50ns (Vds). Both of them have a switching ...
Andr7's user avatar
  • 275
1 vote
1 answer
36 views

How can initial switching artifact lift the entire noise floor?

This switching artifacts occur at first data points. Why does it lift the entire noise floor? Is it because all the frequencies in the white noise got amplitude? or only lower frequencies which is ...
Jtl's user avatar
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0 answers
36 views

What to choose for an RF spectrum analyzer A/D conversion

I assigned myself a (long term) project to build an RF spectrum analyzer. In the basis it will be a super or triple (depending on frequency band) heterodyne frontend with digitized IF and some final ...
Waldorf's user avatar
  • 221
1 vote
1 answer
106 views

Unexpected signal at AD620 module output when Vsupply is on

I bought an AD620 instrumentation amplifier module and below is the module schematic. Souurce - https://protosupplies.com/product/ad620-instrumentation-amplifier-module/ A problem shows up when I'm ...
Jack Huang's user avatar
2 votes
4 answers
812 views

Are these low frequency FFT components due to insufficient op-amp slew rates?

In my circuit I have three op-amps in (direct) series connection: AD8422 acting as an input buffer (a large input impedance is needed) with a fixed gain (slew rate: 0.8 V/us) LTC6910 being used as a ...
Maxim's user avatar
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-1 votes
1 answer
67 views

Working on 64-point SDF FFT Verilog code [closed]

I am an undergraduate student who has chosen to work on an FPGA project for my capstone. My professor provided me with a PDF file without any context, and now I am trying to figure it out on my own. I ...
swnh's user avatar
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1 vote
1 answer
68 views

Discrepancy Between Bode Plot Predictions and FFT Output for Higher-Order Low Pass Filters on a 10 kHz Square Wave

I have conducted an experiment where a 10 kHz square wave is input through low pass filters of 1st, 2nd, 3rd, and 4th orders. After passing the square wave through these filters, I observed the FFT (...
user97662's user avatar
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0 answers
80 views

FFT on my oscilloscope is not detecting a peak which is a peak

I am having a strange problem and it's due to that lack of my info about signals. I examined a PWM output from my MCU under sds1104x-e oscilloscope which is 656KHZ with a duty cycle of around 45.5% ...
abdo Salm's user avatar
  • 143
1 vote
1 answer
1k views

How to plot the FFT in Ltspice?

I'm new in Ltspice and I wonder how to plot the FFT of R1? I just failed to find such a tutorial and I'm sorry for such a stupid question. Thanks for your teaching!
user22650506's user avatar
6 votes
2 answers
967 views

FFT is not showing noise

I have current values and respective time. There is noise in the signal. But when I am calculating FFT, it is not showing I have recorded the values at every 5ms. below is my code: ...
Teena's user avatar
  • 101
1 vote
1 answer
109 views

VHDL my FFT design doesn't work at larger N-point

I built an FFT module for an 128-point FFT in VHDL. When I use the same principle for 8-point fft or 16-point fft (ofcourse then the file must be altered for less stages and different twiddle factors),...
Mart's user avatar
  • 376
1 vote
0 answers
62 views

How do you find Image Rejection Ratio from doing FFT on ADC samples?

I'm trying to understand a particular code written by a former employee for measuring performance of an RX chain in a transceiver. I have to measure IRR(Image Rejection Ratio). Certain input RF ...
Siddaram's user avatar
-2 votes
1 answer
110 views

What two laws of harmonics are demonstrated here? [closed]

I clipped the top and bottom of a sine wave at exact reciprocal angles (73.73159173° to 106.2684083° and 253.73159173° to 286.2684083°) to prevent the creation of even (red) harmonics (by being ...
Dan Bullard's user avatar
0 votes
1 answer
135 views

How would you compute Fourier transform of a real world signal where the signal keeps getting updated (not a static one)?

A very naive question: How do we use Fourier transform for real world signals - for which you have the information only up to the present instant (and the present time keeps moving continuously)? The ...
Dynamic_equilibrium's user avatar
0 votes
2 answers
791 views

Error with arduinoFFT library

I'm trying to use the arduinoFFT library (https://github.com/kosme/arduinoFFT) with my ESP32. I made a basic program just to see if the FFT is done correctly. The problem is that I can't compile the ...
C_Cruz's user avatar
  • 1
8 votes
3 answers
936 views

How to achieve coherent sampling in practice

In the context of measuring a sinusoidal source with an ADC, coherent sampling (i.e. capturing an integer number of periods) allows the use of a rectangular window without spectral leakage. How does ...
DavidG25's user avatar
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0 votes
1 answer
261 views

Fourier Transform of Voltage Readings

I am working on developing a Fourier transform for voltage readings. The goal is to determine voltages at different frequencies (frequency domain). I am taking readings at 100Hz and am measuring both ...
GGonzales0603's user avatar
0 votes
0 answers
102 views

FFT analysis in LTSpice is giving wrong results during partial execution of time domain simulation (Details in body)

In LTspice, while computing FFT using waveform window (View-> FFT), the results seems to be wrong if FFT is computed after partial execution of simulation ( eg: if simulation is paused in between)...
Dynamic_equilibrium's user avatar
0 votes
0 answers
47 views

Data-set creation and signal processing

I'm taking a digital signal processing course and I have a project to create a data set with a microcontroller and process it for this course. I want to count only dog bark for example not dog growl. ...
karl0101's user avatar
0 votes
0 answers
66 views

Propagation phenomena in cables

I have came across the propagation phenomena that takes place when we have a source and a load connected through a cable. We have an impedance mismatch and hence a propagation phenomena. So what I did,...
Wallflower's user avatar
5 votes
2 answers
1k views

Sampled 100 Hz sinusoidal signal from Hall sensor shows 10 Hz sidelobes when sampled at 500 Hz

I am working on a project where I need to measure an oscillating magnetic field that oscillates at a frequency of 100 Hz. For this reason, I am using a Hall sensor. I programmed the Hall sensor to ...
Mantabit's user avatar
  • 343
0 votes
1 answer
196 views

Battery-powered FFT project

I want to use FFT on a STM32F401C01CCU6. I chose this microcontroller because everyone says the F4 series is good at signal processing. I want to make a project that will work with a battery; I am not ...
mrneedshelpalot's user avatar
0 votes
1 answer
356 views

In LTspice, why doesn't the FFT plot update after a simulation?

I'm using LTspiceXVII (17.0.35.0). I run a simulation and make an FFT plot. Then I change a parameter and the time waveform updates, but the FFT plot doesn't update. I have only been able to see ...
Sanjo's user avatar
  • 1
-1 votes
1 answer
49 views

Second-order LPF not filtering correctly

I am trying to implement a 2nd-order Butterworth low-pass filter with a cut-off frequency of 9 kHz. As an input I was using a square wave of 12 kHz. My op-amp rails are 0 and 5 V. The sqaure wave has ...
Andrew Allkin's user avatar
0 votes
0 answers
201 views

Can I directly use an oscilloscope's FFT function for EMI measurement?

The latest oscilloscopes with FFT and real-time functions are very useful for EMI debugging with a near-field probe, but why can't we just use them for the far-field EMI scan? EMI receivers are much ...
David W's user avatar
3 votes
1 answer
925 views

How to have a wider spectrum of FFT at LTspice?

I've used the FFT analysis tool to see the spectrum of DM and CM EMI noise in the range of 150K to 30MHz. But the analysis stops at about 3MHz automatically and there is no option to set the max and ...
WeTech's user avatar
  • 735
1 vote
2 answers
146 views

Filtering when collecting a signal in the time domain

I reconstructed a signal (from simulations) in the time domain from t ~ 0.01 to 2000 s. I do this by calculating "modes" or constants for the system that I'm studying that should be exact ...
user avatar
3 votes
2 answers
2k views

How to remove aliasing effects?

I collected data in the time domain and then calculated the FFT to see how it looks in the frequency domain. Since the signal isn't band-limited, I think I have aliasing and there are features at high ...
user avatar
0 votes
1 answer
87 views

How do you bandlimit a continuous-time signal?

I wish to avoid aliasing as a result of taking the FFT of a signal. The signal isn't band limited. My understanding is that I should band limit the continuous-time signal first. How do I do this? EDIT:...
user avatar
1 vote
0 answers
151 views

Range Profile in Radar System

I am slightly confused on the range profile I am obtaining from my SCFW radar dataset. I am understanding of how fftshift works in shifting positive and negative frequency components to be centered ...
Victor Ramirez's user avatar
0 votes
1 answer
257 views

Generating a sinewave on Matlab with specific harmonics [closed]

I need to generate a sinewave on Matlab so I can use the generated values, I need 1000 samples per cycle for grid simulation using micro-controller. I was able to generate a pure sinewave and it is ...
BSaleh's user avatar
  • 23
0 votes
0 answers
96 views

What is the unit of power spectral density?

I have an oscilloscope data, I did plot in time domain, so that unit is in volts (Y axis). After that I took FFT. So now what will be the unit in Y axis? If I will do 10 log 10 (y axis) can it is ...
ARATHI P's user avatar
4 votes
0 answers
265 views

Creating a Range Profile

I am relatively new to the radar/signal processing community. However, my team has been struggling in developing a range profile in our programming. Here are the details for a NI LabVIEW forum I ...
Victor Ramirez's user avatar
0 votes
0 answers
172 views

How to get audio frequencies from mems microphone in PC oscilloscope

I'm fairly new to electronics/signal processing and I'm stuck on trying to obtain frequencies from an Adafruit SPW2430 MEMS microphone. I tried plugging the microphone to a Hantek 6022BE PC ...
felicianomiro's user avatar
2 votes
2 answers
306 views

Generating and measuring a sine wave with THD below -80dB

For an experiment I need to generate a sine wave which has a THD below -80dB. I would like to do that using a band-pass filter connected to a function generator because the output of the function ...
Mantabit's user avatar
  • 343
1 vote
0 answers
233 views

Cross-correlation in time or frequency domain (FPGA)

I need to compute the complex cross-correlation between two signals a and b and the take only the maximum value of the cross-correlation, regardless of the lag time where this occurs. I can do it ...
Albert's user avatar
  • 11
3 votes
3 answers
235 views

LTSPICE: Dirty gate voltage and low gain - Hartley oscillator using MOSFET

I'm designing a Hartley oscillator using a MOSFET, but I have some issues. I've chosen L1 as 3 times L2 to obtain a gain of 3, however, it's more like 2.4. First of all, I can't understand why the ...
nor_gate's user avatar
2 votes
1 answer
1k views

.four plot FF/t LTspice

I am trying to plot an FFT in LTspice using a .four command and the spice error log directly without using the FFT on LTspice (right click + view + FFT once the ...
George's user avatar
  • 87
1 vote
0 answers
163 views

What determines amplitude resolution of oscilloscope FFT?

I am studying my oscilloscope's FFT measurement capability (Rigol DS1054Z. Links to datasheet and user guide), and do not understand what determines the amplitude resolution capability of the FFT ...
user avatar
0 votes
2 answers
426 views

What is happening when a signal is acquired at exactly Nyquist frequency?

So I am doing some some simulations with 1V sinusoidal signals, and a 1 second acquisition at sampling frequency of 100 kHz. Now I am slowly increasing the frequency of the sinusoidal signal. Trying 1 ...
Granger Obliviate's user avatar
0 votes
1 answer
237 views

Windowing FFT: spectral leakage vs ENOB

I am using Cadence to perform full speed testing of a 4-bit SAR ADC using the FFT and non-coherent sampling. I've obtained the ENOB and the spectra of the output signal using first the rectangular ...
Granger Obliviate's user avatar
0 votes
1 answer
130 views

Why am I getting linear phase?

I'm trying to plot the phase of Fourier transform of the function below: \begin{gather*} x[n] = 2e^{-0.9|n|} \: \: n \in [-5,5]\\ x[n] = 0 \:\: n \notin [-5,5] \end{gather*} which is equal to the ...
AmirHossein Bagheri's user avatar
1 vote
0 answers
149 views

Numpy giving the wrong Fourier transform of sine functions

I'm using numpy's FFT to calculate the Fourier transform of \$\cos(\omega_0t)\$ and plot it. By using the definition we can derive its Fourier transform as below: \begin{equation} X_3(j\omega) = \pi (...
nothatcreative5's user avatar
0 votes
1 answer
701 views

How can we obtain the input signal given the output signal and transfer function?

From an electrical circuit, I have the transfer function as: $$ H(s) = \frac{R}{sRC + 1} $$ \$V\$ is the output and I is the input so we can write: $$ V(s) = I(s) H(s) $$ or $$ V(jω) = I(jω) H(jω) $$ ...
GNZ's user avatar
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